Ultrasonic inspection employs high frequency, ultrasonic waves generated by a transducer to examine test objects and make measurements. Ultrasonic inspection can be used for detection of flaws in test objects, and for performing evaluation, dimensional measurements, material characterization, and more on objects. Measurement procedures initially developed for metals have been extended to engineered materials such as composites, where such characteristics as anisotropy and inhomogeneity are of concern. Advances in digitization and computing capabilities have changed the types of instruments and algorithms that are used in processing the resulting data. High-resolution imaging systems and multiple measurement modalities for characterizing a flaw have emerged. Of interest are detecting, characterizing, and sizing defects, as well as characterizing the materials in which they are found. The objectives of ultrasonic testing range from the determination of fundamental microstructural characteristics such as grain size, porosity, texture and preferred grain orientation, to material properties related to failure mechanisms such as fatigue, creep, and fracture toughness.
In ultrasonic testing, a transducer containing a piezoelectric element is excited by an electrical pulse to transmit an ultrasonic pulse into a test object. The sound wave propagates through the test object and is reflected. The transducer receives the reflected wave and the reflected wave is converted by the transducer into an electrical signal and analyzed to determine whether a discontinuity exists in the test object. A flaw or discontinuity in the test object is characterized by certain anomalous profiles in the electrical signal that are viewable on an analog display, such as an oscilloscope or a recording device.
The electrical signal from a transducer representing the reflected sound wave must be amplified for input into a display or recording device. In order to be displayed, the amplified signal must fall within a certain dynamic response range that is defined by the maximum and minimum operating parameters of the respective display device. In many cases, the signal representing the acoustic wave includes multiple components. For example, in the case of a flaw occurring near the surface of an object, the echo caused by the near-surface defect is received within the interval of the initial pulse. In that case the flaw is not detectable by a visual display, or a recording device, since the amplitude of the echo is small relative to the initial pulse. Since the instrument receives them concurrently, the relatively small-magnitude echo signal from the near-surface flaw is essentially superimposed on the larger initial pulse or interface signal. The large difference in amplitude of the two simultaneous signals makes the smaller signal very difficult to detect.
Previously, analog logarithmic amplifiers have been used in attempts to compress the dynamic response range, but analog amplifiers are not particularly well suited to perform this due to the limited bandwidth and dynamic response range of analog logarithmic amplifiers. Analog techniques are limited by noise and accuracy problems.
Therefore there is a need for a device for digitally processing simultaneous ultrasonic signals having widely varying amplitudes and combining them for display into a continuous linear digital signal having a wide dynamic range.